Lets consider a
1D (e.g. Audio) example to see what the different domains mean:

Consider a complicated sound such as the noise of a car
horn. We can describe this sound in two related ways:

sample the amplitude of the sound many times a second, which gives an
approximation to the sound as a function of time.

analyse the
sound in terms of the pitches of the notes, or frequencies, which
make the sound up, recording the amplitude of each frequency.

In the example below (Fig ) we have a signal that consists of a sinusoidal wave at 8 Hz.
8Hz means that wave is completing 8 cycles in 1 second and is the frequency of that
wave. From the frequency domain we can see that the composition of our signal is one
wave (one peak) occurring with a frequency of 8Hz with a magnitude/fraction of 1.0
i.e. it is the whole signal.